Study On Upgrading Of Straw Briquettes And Emission Characteristics Of Sulfur And Nitrogen During Combustion

Attaching great importance to developing biomass energy is an important measure to adjust the energy structure and relieve the pressure of environmental pollution in China.As a large agricultural country,China has abundant biomass resources,especially straw biomass resources.However,due to its wide distribution range,low energy density and low calorific value of fuel,the transportation and storage costs are greatly improved,limiting the large-scale application of straw biomass fuel.Although the low sulfur and ash content is the main advantage of biomass,but with the increasingly stringent emission standards,the sulfur and nitrogen pollutant emissions during the thermochemical conversion process cannot be ignored like before.In addition,the problem of ash melting during the process of heat transfer and combustion because of the high content of alkali metal also needs to be solved.At present,the densification process,quality improvement and additive modification are effective methods to solve such defects as mentioned before.Therefore,for the purpose of biomass fuel upgrading and its large-scale clean utilization,in this paper,the sulfur and nitrogen release characteristics of biomass briquettes are investigated by experiment and theoretical analysis.Moreover,the effects of additives,temperature and other factors on the biomass briquette desulfurization characteristics,nitrogen emission characteristics and ash melting effect are also studied.The main works of the dissertation are listed as following:(1)The biomass briquettes were prepared by a laboratory briquetting equipment,and its physical properties and combustion characteristics were investigated.The micromorphology of briquette showed that forming mechanism was influenced by lignin content Maize straw with low lignin content was linked together mainly depends on the bridging effect of each part,and the high lignin content of rice straw mainly depended on its adhesion effects after the softening of lignin.Due to the different forming mechanism,the volume density of rice straw in each molding pressure conditions were higher than that of maize straw and the water resistance is better than that of maize straw;both anti-breaking performance were improved along with the forming pressure increased,higher than 15 MPa the performance will be good.The combustion process of biomass briquette is similar to powder,but the volatilization rate of devolatilization was reduced by the briquetting process.With the increase of briquetting pressure,the ignition temperature increased,the maximum weight loss rate reduced,combustion characteristic index reduced and the combustion performance deteriorates.(2)The sulfur transformation and sulfur emission characteristics of biomass briquettes were conducted in a lab-scale tubular reactor.The experimental results showed that compared with biomass powder biomass briquettes had excellent sulfur self-retention properties.At 900 ?,five kinds of biomass briquettes(PS/PW/CS/RS/MS)sulfur self-retention ratio reached above 90%;at 1000 ?,the effect of self-retention of sulfur is more obvious,mainly because the amount of sulfur released during devolatilization was greatly reduced.The sulfur retention ratio of woody plant(ps/pw)can be increased to 96.00%and 95.59%,respectively.The sulfur retention ratio of herbaceous plants(ca/ra/ma)can be increased to 82.92%.87.90%?88.16%,respectively.An appropriate quantity of FG coal was mixed in the briquette for its upgrading,not only can improve the calorific value of biomass briquettes,but also play a role in sulfur retention by generating sulfate with better thermal stability(BaSO4/SrSO4).In the range of 700 ? to 1100 ?,the sulfur release ratio increased as the temperature increases.Below 900 ?,the release ratio was kept under 5%and steeply increased after 900 ?.Above 900 ?,the K/Ca in biomass preferably incorporated into glassy silicate matrix instead of forming sulfates,and the higher the temperature,the more obvious the trend of this reaction.Different additives showed different enhancement effect of sulfur retention,including organic additives(Calcium sorbate,CS1/Calcium stearate,CS2)and inorganic additives(CaCO3/CaO/K2CO3).The experiment results showed that the potassium based additives(K2CO3)was better than that calcium based additives,organic calcium additives(CS1/CS2)was better than inorganic based ones(CaCO3/CaO).Among all the additives,the CS1 had ideal effect,which improve the sulfur retention ratio by the formation of more stable sulfates,such as Ca3(SO3)2SO4,Ca3(SO3)2.12(SO4)0.88?K2Ca(SO3)2?K2Ca2(S04)3?K2CaMg(SO4)3.At 900? and 1000?,the release amount of SO2 were reduced to 0.036 g/kg(0.002 g/MJ)and 0.134 g/kg(0.008 g/MJ),respectively,which achieved ultra-low emission standards.Moreover,the addition of CS1 can delay the release time of sulfur and reduce the concentration of SO2 during combustion.(3)In the study of sulfur emission during combustion,the release and emission characteristics of nitrogen oxides were also investigated under the corresponding reaction conditions.In the process of combustion of 5 kinds of biomass fuels,NO had only one release peak in the early stage of devolatilization.Compared with biomass powder,the briquetting process hardly had effect on the NO conversion ratio,but significantly reduced the NO emission concentration,and nitrogen release ratio negatively related to the biomass nitrogen content.In the range of 700 ? to 1100 ?,with the increase of temperature,nitrogen conversion ratio showed a step rising trend.After the addition of different sulfur retention additives,the nitrogen conversion ratio was slightly increased,and the effect of CS1 was the least,at 1000 ?,the nitrogen conversion ratio only increased from 26%to 28%,corresponding to the N02 emission of 9.953 g/kg(0.583 g/MJ),less than 15.385 g/kg(0.634 g/MJ)of Fugu coal under the same reaction condition,which can reduce the subsequent de nitrification pressure.While the negative effect on nitrogen conversion ratio of K2CO3 was relatively obvious,the ratio was increased to 37%,but it had no effect on the NO emission concentration.In view of the effect of sulfur retention properties and nitrogen oxides emission,CS1 could be a suitable sulfur retention additive.(4)In order to improve the fouling and slagging problems caused by high alkali metal content in the process of biomass thermal conversion,ammonium dihydrogen phosphate(ADP)was selected as the main additive to explore its effect on improvement of biomass briquette ash melting characteristics.The experiment results showed that the ash melting point of five kinds of biomass briquettes mixed with FG coal can be improved by adding ADP with P/K molar ratio of 1.5,and the enhancement of ADP on the flow temperature is positively correlated with the K content of biomass.Among all the five biomass,the enhancement effect of ADP on rice straw was most obvious,that all four kinds of ash melting characteristics temperature were above 1500 ? after ADP added.The micro morphology of the ash after combustion and the scanning electron microscope also prove that ADP can improve the ash fusion characteristics.On the basis of ADP,after the addition of sulfur retention additives above-mentioned,the maze straw briquette mixed with coal ash melting point had a certain improvement,no negative effects,in which the improvement of additive CS1 is relatively well.The results of this study provide a theoretical basis for the upgrading of biomass fuel and the control of sulfur and nitrogen contaminants,which is of reference value and practical significance for the further optimization of biomass briquette and large-scale clean usage.